1. Field of the Invention
The present invention relates to a synthetic filament-reinforced polymer material sheet and a process for producing the same. More particularly, the present invention relates to a very thin synthetic filament reinforced polymer material sheet having a very small thickness and an enhanced tensile strength and a process for producing the same with a high productivity.
2. Description of the Related Art
It is well known that steel sheets are useful as a reinforcing material and a packing or crating material. However, the steel sheets are disadvantageous in having a relatively large thickness and weight. Recently, a new material having a reduced thickness and weight and useful, in place of the steel sheet, as a reinforcing material and a packing material has been sought.
For example, to meet these requirements, various types of light sheet materials comprising a core consisting of a fiber web, for example, a woven fabric, and impregnated with a polymer material have been disclosed. However, these conventional fiber web-reinforced polymer material sheets are disadvantageous in that they have an unsatisfactory mechanical strength and a larger thickness than that of the conventional steel sheet.
For example, in a conventional reinforcing sheet consisting of a woven fabric, a number of warp yarns intersect a number of weft yarns so that they are curved around each other in a zigzag form, and each of the warp and weft yarns consists of a large number of individual filaments which are twisted and are in contact with each other.
Accordingly, when the woven fabric is stretched in a direction in parallel to or at right angles to the longitudinal axis of the woven fabric, a tensile force is applied to each of the individual filaments in the warp yarns or weft yarns at inclined angles from the longitudinal axis of the filament. Thus, the tensile strength of each individual filament cannot be completly utilized for the tensile force.
Also, in the woven fabric, the individual filaments are usually in contact with each other. Therefore, it is difficult for the polymer material to evenly penetrate the spaces formed among the individual filaments and to be evenly distributed in the woven fabric. This difficulty causes the resultant reinforced sheet to exhibit an unsatisfactory mechanical strength.
Furthermore, in the woven fabric, the polymer material penetrates mainly into large spaces formed among the warp and weft yarns. This phenomenon causes the polymer material to be unevenly distributed in the woven fabric and the resultant woven fabric-reinforced sheet to exhibit an unsatisfactory mechanical strength.
Still further, the conventional woven fabric is not satisfactory when attempting to provide a very thin reinforced polymer material sheet having a thickness of, for example, 0.25 mm or less.
Additionally, when the conventional woven fabric is used as a reinforcing sheet, the resultant reinforced polymer material sheet has rough surfaces thereof and thus exhibits an unsatisfactory smoothness and evenness.
Another conventional reinforcing sheet consists of a number of multifilament yarns each consisting of a number of individual filaments. The multifilament yarns are arranged in parallel to each other, do not intersect each other, and have straight individual filaments.
Accordingly, the multifilament yarn reinforcing sheet is useful for producing a reinforced polymer material sheet having a higher mechanical strength and a smaller thickness than those of the above-mentioned conventional woven fabric-reinforced polymer material sheet.
However, the individual filaments are bundled in each yarn and are in contact with each other.
Therefore, when impregnated by a polymer material, the polymer material mainly penetrates into the spaces formed between the yarns and it is difficult for the polymer material to penetrate between the individual filaments. Thus, the polymer material is unevenly distributed in the reinforcing sheet and some of the spaces between the individual filaments are not filled by the polymer material. Thus the resultant multifilament yarn-reinforced sheet exhibits an unsatisfactory mechanical strength, is easily split along the spaces not filled by the polymer material, and has poor durability in use.
Also, since the individual filaments in each yarn are bundled but not opened, it is difficult to provide a very thin multifilament yarn-reinforced polymer material sheet having a thickness of, for example, 0.25 mm or less, unless very thin multifilament yarns are used. Note, very thin multifilament yarns are very expensive. Additionally, the multifilament yarn-reinforced polymer material sheet has rough surfaces and exhibits an unsatisfactory smoothness and evenness.
Accordingly, the conventional fiber web-reinforced or multifilament yarn-reinforced polymer material sheets are not always usable in place of the steel sheet.
Attempts have been made to use other sheets, for example, light metal sheets such as Duralumin sheets and synthetic polymer sheets, in place of the steel sheets. However, these sheets are disadvantageous in that they have a large thickness and/or low mechanical strength and durability in use, and therefore, the attempt has not always been successful.